Amorphous alloys, when properly formed from the molten state at sufficiently fast cooling rates, have high elastic limits, typically in the range of from 1.8% to 2.2%. Further, these amorphous alloys may show substantial bending ductility of up to 100%, such as in the case of thin melt spun ribbons. In addition, amorphous alloys being capable of showing glass transition are further capable of forming a super-cooled liquid above the glass transition range and can be significantly deformed using very small applied forces (normally, 20 MPa or less).
Recently bulk-solidifying amorphous alloys have been discovered which can be cooled at cooling rates of about 500 K/sec or less from their molten state to form objects of 1.0 mm or more thickness with substantially amorphous atomic structure. These bulk-solidifying amorphous alloys are substantially thicker than conventional amorphous alloys, which have thicknesses of typically 0.020 mm, and which require cooling rates of 105 K/sec or more. U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975 (each incorporated by reference herein) disclose such families of bulk solidifying amorphous alloys. The discovery of bulk-solidifying amorphous alloys gives rise to a wide-variety of applications. As such, a practical and cost-effective method of forming bulk-solidifying amorphous alloys, such as molding around the glass transition range, is desired to allow for the use of these materials in designs requiring intricate precision shapes. It should be noted that substantial bending ductility (as much as 100%) is not necessarily essential for all applications of bulk-solidifying amorphous alloys—as they are designed to utilize elastic limit—although at least some percent of bending ductility is generally preferred.
U.S. Pat. Nos. 6,027,586; 5,950,704; 5,896,642; 5,324,368; and 5,306,463 (each incorporated by reference herein) disclose methods of forming molded articles of amorphous alloys exploiting their capability of showing a glass transition. However, it has been recently observed that amorphous alloys may lose their ductility when subjected to temperatures around the glass transition temperature. Indeed, a substantial portion of the high elastic limit of most bulk-solidifying amorphous alloy may easily be lost during these conventional forming processes, even though the amorphous material itself may substantially retain its amorphous structure. Beyond the loss of the elasticity of the final product, these methods may also lead to a loss of fracture toughness, which limits the ultimate strength levels attainable with the material. Indeed, the loss of high elastic limit becomes the norm rather than the exception utilizing conventional methods of forming molded articles of bulk solidifying amorphous alloys. Although this phenomenon has been attributed to a variety of factors, such as micro-crystallization and structural relaxation, a variety of thermally activated processes—such as spinodal decomposition and formation of nano-crystals—may also be at least partially responsible. U.S. Pat. Nos. 5,296,059 and 5,209,791 (each incorporated by reference herein) try to address the loss of substantial bending ductility and disclose methods of imparting ductility to amorphous alloys subjected to temperatures around the glass transition range. Despite these attempts, no prior art method of forming bulk-solidifying amorphous alloys adequately addresses the problem of lost ductility and high elastic limit.
For example, after practicing various molding process of bulk-solidifying amorphous alloys around the glass transition range, the elastic limit may become as small as 0.1% even though the alloys are deemed substantially amorphous by conventional methods such as X-ray diffraction. Moreover, X-ray diffraction techniques, commonly used to determine amorphous structure in prior art methods, prove to be insufficient for quick and cost-effective—if effective at all—detection of loss in elastic limit, although it shows substantially amorphous structure.
In essence, the prior art methods of forming molded articles of amorphous alloy do not generally preserve the high elastic limit of bulk-solidifying amorphous alloys after the forming and shaping process has been completed. Accordingly, a new and improved method of forming molded articles of bulk solidifying amorphous alloys is desired, which substantially preserves the high elastic limit upon completion of molding process.